The field of the invention relates to hydrostatic transmissions in association with transaxle driving apparatus for use in light grass-mowing tractors, and where in particular it becomes advantageous to manufacture the hydrostatic transmission and transaxle as one unitary item.
The present invention is directed towards providing improvements to hydrostatic variable speed transaxles of the type as shown in U.S. Pat. No. 4,979,583 entitled "Variable Speed Transaxle".
The prior application has disclosed therein an embodiment for a hydrostatic transmission wherein a vertically aligned input drive shaft of the transaxle is in driving engagement with the rotary cylinder member of the hydrostatic pump, and where the hydrostatic motor is connected via a gear train that includes bevel gearing, to the differential and axle drive shafts of the transaxle.
Pump and motor are fluidly coupled together by a valve member, the valve member being either supported directly by the housing of the transaxle, or by means of an intermediary plate.
In order to reduce noise and vibration from certain internal components, such as the valve member, in addition to also obtaining the most power efficient operating range for the hydrostatic transmission, it is preferrable to limit the maximum rotational speed of the pump and motor to a level below that of the engine.
This can be achieved by gear reduction as disclosed in U.S. Pat. No. 4,691,512, or through the use of two unequal sizes of pulley in a Vee belt drive train. The Vee-belt being used to transmit power from the vehicle's engine to the transaxle, speed reduction being obtained by the use of a small pulley on the engine shaft driving, by the Vee belt, a larger pulley on the transaxle input shaft. The degree of speed reduction being determined by the the ratio of the pulley diameters.
In the field of hydrostatic transmissions, speed reduction by Vee belt rather than by gearing has become widely used by the industry for economic reasons.
A typical grass-mower lawn tractor has a maximum forward speed of about 6 miles per hour, and is most frequently fitted with an internal combustion engine operating at a rotational speed of about 3,400 rev/min at full throttle. For reasons of efficient grass cutting, it is necessary to operate the cutting blades at about the same rotational speed as the engine.
To obtain a maximum vehicle forward speed of 6 mph, speed reduction is required between the engine and the drive wheels, the maximum rotating speed of the drive wheels being typically of the order of 110 rev/min. A proportion of this speed reduction being obtained by the Vee belt drive-line as already described, so that less final gear reduction is required inside the transaxle.
During the test and development of such a Vee belt drive system in a lawn tractor fitted with a prototype hydrostatic transaxle, it has be found that on occasion insufficient engine torque can be transmitted through the Vee belt to the hydrostatic transaxle, and furthermore, that the Vee belts wear out prematurely. Also the required tension in the Vee belt is such that during prolonged use, the belt becomes loose with the result that the transmittable power to the transaxle is further reduced.
A further disadvantage of obtaining a large speed reduction with a Vee belt drive, is that the cooling fan attached to the input shaft of the transaxle is rotating at less than engine speed. This can be a severe disadvantage during extremely hot spells in the summer as the oil viscosity of the power transmitting hydraulic fluid in the hydrostatic unit drops with temperature rise, causing a corresponding fall in speed performance of the tractor.
It has further been found that if the tension in the Vee belt is reduced to a point where it is slack by the engagement of the clutch pedal, a feature originally developed to satisfy a general market requirement to enable the tractor to be pushed manually, there is still sufficient belt contact with the drive pulley to prevent easy belt slipping.
What is needed in the art is an improved transaxle assembly with a hydrostatic transmission for use with a vertical aligned input shaft operating at approximately the same rotational speed as the engine for reasons of both efficient torque transmission and fan cooling, with speed reducing bevel gearing disposed between the input shaft and the hydrostatic pump for improved operation. What is further needed is positive locking means for securing the hydrostatic pump and motor to the housing in order to reduce noise and vibration, an effective fluid circuit breaker means to enable the vehicle to be manually pushed without engaging the engine, and a hydrostatic control shaft that can be coupled to either side of the tractor without difficulty.